Charge controllers are sized based on the solar array’s current and the solar system’s voltage. You typically want to make sure you have a charge controller that is large enough to handle the amount of power and current produced by your panels. If your solar system’s volts were 12 and your amps were 14, you would need a charge controller that had at least 14 amps. However due to environmental factors, you need to factor in an additional 25%. This brings the minimum amps that this charger controller must have to 17.5 amps. In this example, you would need a 12 volt, 20 amp charge controller.